Recently, there has rapidly been developed an antenna for reception of satellite broadcasting which is to be mounted to a moving body such as sightseeing bus, personal vehicle and RV (recreational vehicle). In this kind of vehicle-mounted antenna, the direction of a broadcasting satellite (BS) seen from the antenna changes momentarily with a change in route of the vehicle or the like. Therefore, it becomes necessary to perform a tracking operation for controlling the azimuth angle and the elevation angle of the antenna so that the antenna is always directed towards the broadcasting satellite. As a result, it is required that a rotating coupler for allowing the antenna to make a relative rotation while maintaining the electrical coupling of a signal frequency band should be provided a feeder line which connects the antenna and a tuner. Such a rotating coupler may include a high frequency type rotating coupler which is provided between a rotating antenna and a stationary converter in order to couple a receive signal having a frequency in the vicinity of 12 GHz. In another type of rotating coupler, an antenna and a converter are integrated with each other and a receive signal having a frequency in the vicinity of 12 GHz is converted once by the converter into an intermediate frequency signal having a frequency of about 1 GHz. This type of rotating coupler or a low frequency type rotating coupler is provided a transmission path of the intermediate frequency signal. Both types of rotating couplers have their merits and demerits. But, the low frequency type rotating coupler is regarded as being advantageous with respect to electrical characteristics such as S/N ratio and frequency characteristic.
The low frequency type rotating coupler as mentioned above has a structure shown in FIGS. 4A and 4B. As shown in FIG. 4A, a coupling plate 30 includes an insulating plate 31, a non-grounded (or hot side) conductor plate 32 formed on one of opposite surfaces of the insulating plate 31, a grounded conductor plate 33 formed on the other surface thereof, and a series connection of an impedance matching resistor R1 and a DC blocking capacitor C1 provided between the non-grounded conductor plate 32 and the grounded conductor plate 33. Reference numeral 34 denotes a conductor plate which is formed on the one surface of the insulating plate 31 so as to enclose the non-grounded conductor plate 32. One terminal of the capacitor C1 is connected to a conductor which extends through the insulating plate 31 and is connected to the non-grounded conductor plate 32 formed on the one surface of the insulating plate 31. As shown in FIG. 4B, the coupling plate 30 and a coupling plate 40, having the same structure as the coupling plate 30, are arranged apart from each other and opposing each other so that a coupling capacitance is formed by the non-grounded conductor plates 32 of the coupling plates 30 and 40 and a gap provided therebetween. Coaxial connectors 35 and 45 are connected to the coupling plates 30 and 40, and the coupling plates are rotatably held to face each other by holding mechanisms (not shown) provided on the peripheral portions.
FIG. 5 shows an equivalent circuit of the rotating coupler having the structure shown in FIGS. 4A and 4B. A coupling capacitance C2 is formed by the non-grounded conductor plates of the two coupling plates and a gap provided therebetween, and a coupling capacitance C3 is formed by the grounded conductor plates of the two coupling plates and a gap provided therebetween. The coupling capacitance C3 includes a series connection of a coupling capacitance formed by the grounded conductor plate 33 and the conductor plate 34 of one of the two coupling plates and the insulating plate 31 interposed therebetween, a similar coupling capacitance formed by the other coupling plate, and a coupling capacitance formed between the two conductor plates 34. In each coupling plate, the electrostatic capacitance C1 or C4 of the DC blocking capacitor and the resistor R1 or R2 are connected in series with each other between the non-grounded conductor plate and the grounded conductor plate. Reference symbol Ro denotes an output resistor on the converter side, and symbol RL denotes a load resistor on the tuner side.
A problem encountered in the non-contact rotating coupler having the structure shown in FIGS. 4A and 4B and the equivalent circuit shown in FIG. 5 is how to reduce a coupling loss. It is therefore required that the coupling capacitances C2 and C3 be made sufficiently large. In order to make the coupling capacitance sufficiently large, it is necessary not only to make the area of each conductor plate sufficiently large but also to make an interval between the two coupling plates sufficently narrow. However, aside from the grounded conductor plate, there is a limit to enlargement of the area of the non-grounded conductor plate formed at the central portion. Also, the reduction of the interval between the coupling plates has a limit from the mechanical precision and stability point of view.